1. Field of the Invention
The present invention relates to wavelength division multiplexing optical communication, and more particularly to an optical signal transmission apparatus including a reflective gain-clamped semiconductor optical amplifier and an optical communication system using the optical signal transmission apparatus.
2. Description of the Related Art
Generally, a wavelength division multiplexing passive optical network (hereinafter, referred to as a WDM PON) provides an ultra high-speed broadband communication service using specific wavelengths assigned to each subscriber terminal. Therefore, the WDM-PON can (1) ensure the secrecy of communication, (2) easily accommodate special communication services required from each subscriber terminal or enlargement of channel capacity, and (3) easily increase the number of subscribers by adding specific wavelengths to be assigned to new subscribers. However, in spite of these advantages, the WDM-PON has not yet been put to practical use. This is because the WDM-PON requires both a light source having a specific oscillation wavelength and an additional wavelength stabilization circuit to stabilize the wavelength of the light source in the central office (CO) and each of the subscriber terminals. This, in turn, places a heavy economic burden on subscribers. Therefore, there is a need for an economical wavelength-division-multiplexed light source for use with the WDM-PON.
A number of devices have been proposed as the wavelength division multiplexing light sources. Such devices include, distributed feedback laser arrays (hereinafter, referred to as DFB laser arrays), multi-frequency lasers (hereinafter, referred to as MFLs), spectrum-sliced light sources, and mode-locked fabry-perot lasers with incoherent light, etc.
However, the DFB laser array and the MFL have complicated manufacturing processes. In addition, they are high-priced devices in which correct wavelength selectivity and wavelength stabilization are necessary for the wavelength-division-multiplexed method.
Spectrum-sliced light sources have been studied recently. These devices spectrum-slice an optical signal of wide bandwidth using an optical filter or a waveguide grating router (WGR) to provide a large number of wavelength-divided channels. Thus, the spectrum-sliced light source does not require a light source having a specific oscillation wavelength nor a device to stabilize wavelength. A number of devices have been proposed as the spectrum-sliced light sources. These devices include, light emitting diodes (hereinafter, referred to as LEDs), super luminescent diodes (hereinafter, referred to as SLDs), fabry-perot lasers (hereinafter, referred to as FP lasers), fiber amplifier light sources, and ultra-short pulse light sources, etc.
The LED and SLD, which have been proposed as the spectrum-sliced light sources, have very wide optical bandwidths and low prices. However, they have narrow modulation bandwidths and low output power. Thus, they have characteristics better suited for a light source for upward signals which have lower modulation speed as compared to downward signals. The FP laser is a high-power device with a low-price. However, it has a narrow bandwidth, thus cannot provide a number of wavelength-divided channels. Also, with the FP laser, serious degradation is caused by mode partition noise, when modulating a spectrum-sliced signal at high speed and transmitting the modulated signal. Further, the ultra-short pulse light source has a spectrum bandwidth of the light source that is very wide and has coherence. However, it has disadvantages including stabilization of the spectrum to be oscillated, which is low and has a pulse width only a few ps. Thus, realization of such an ultra-short pulse light source is difficult.
In addition to these light sources, spectrum-sliced fiber amplifier light sources have been proposed. A spectrum-sliced fiber amplifier light source, spectrum-slices an amplified spontaneous emission light (ASE light) generated from an optical fiber amplifier and then provides a number of wavelength-divided high-power channels. However, such light sources must use a high-priced external modulator such as an LiNbO3 modulator, so that each channel transmits different data from each other.
A mode-locked FP laser with incoherent light spectrum-slices a wide bandwidth optical signal. The optical signal is generated from an incoherent light source, such as a light emitting diode or a fiber amplifier light source. The mode-locked FP laser uses an optical filter or a waveguide grating router to slice the optical signal. It then inputs the spectrum-sliced light signals into an FP laser having no isolator. Subsequently, a mode-locked signal outputted from the FP laser is used for transmission. When a spectrum-sliced signal, above a predetermined output power, is inputted into an FP laser, the FP laser generates and outputs only the same wavelength as that of the spectrum-sliced signal input to the FP laser. The mode-locked FP laser with incoherent light transmits data economically, since it directly modulates an FP laser according to a data signal.
However, when the spectrum-sliced incoherent light does not coincide with an Fabry-Perot mode, the mode-lock phenomenon of the FP laser is released. Accordingly, when the FP laser is used as an optical transmitter, a central wavelength of an incoherent light source must always coincide with the Fabry-Perot mode, or predetermined wavelength intervals must be maintained. This is required in order to maintain stable transmission characteristics even with respect to variation of environment such as temperature.
A reflective semiconductor optical amplifier (hereinafter, referred to as a R-SOA) has been proposed. When the R-SOA is used, since the gain property of the R-SOA does not rapidly change according to wavelength, it is unnecessary to coincide a spectrum-sliced light source with a mode. Also, intensity noise of incoherent light source is suppressed using the gain saturation property of the amplifier according to the input intensity of the purposely spectrum-sliced light source. Thus, an increased modulation speed can be obtained.
However, in the R-SOA, gain saturation according to gain intensity occurs smoothly, and thus suppressing intensity noise by gain saturation is limited.